On Thu, Sep 7, 2023 at 11:44 AM Alexander Larsson <alexl@xxxxxxxxxx> wrote: > > There are cases where you want to use an overlayfs mount as a lowerdir > for another overlayfs mount. For example, if the system rootfs is on > overlayfs due to composefs, or to make it volatile (via tmps), then > you cannot currently store a lowerdir on the rootfs. This means you > can't e.g. store on the rootfs a prepared container image for use > using overlayfs. > > To work around this, we introduce an escapment mechanism for overlayfs > xattrs. Whenever the lower/upper dir has a xattr named > `overlay.overlay.XYZ`, we list it as overlay.XYZ in listxattrs, and > when the user calls getxattr or setxattr on `overlay.XYZ`, we apply to > `overlay.overlay.XYZ` in the backing directories. Please use "" > > This allows storing any kind of overlay xattrs in a overlayfs mount > that can be used as a lowerdir in another mount. It is possible to > stack this mechanism multiple times, such that > overlay.overlay.overlay.XYZ will survive two levels of overlay mounts, > however this is not all that useful in practice because of stack depth > limitations of overlayfs mounts. > > Signed-off-by: Alexander Larsson <alexl@xxxxxxxxxx> With comment in ovl_listxattr() below fixed you may add: Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx> > --- > fs/overlayfs/overlayfs.h | 7 ++++ > fs/overlayfs/xattrs.c | 78 +++++++++++++++++++++++++++++++++++++--- > 2 files changed, 81 insertions(+), 4 deletions(-) > > diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h > index dff7232b7bf5..736d7f952a8e 100644 > --- a/fs/overlayfs/overlayfs.h > +++ b/fs/overlayfs/overlayfs.h > @@ -32,6 +32,13 @@ enum ovl_path_type { > #define OVL_XATTR_USER_PREFIX XATTR_USER_PREFIX OVL_XATTR_NAMESPACE > #define OVL_XATTR_USER_PREFIX_LEN (sizeof(OVL_XATTR_USER_PREFIX) - 1) > > +#define OVL_XATTR_ESCAPE_PREFIX OVL_XATTR_NAMESPACE > +#define OVL_XATTR_ESCAPE_PREFIX_LEN (sizeof(OVL_XATTR_ESCAPE_PREFIX) - 1) > +#define OVL_XATTR_ESCAPE_TRUSTED_PREFIX OVL_XATTR_TRUSTED_PREFIX OVL_XATTR_ESCAPE_PREFIX > +#define OVL_XATTR_ESCAPE_TRUSTED_PREFIX_LEN (sizeof(OVL_XATTR_ESCAPE_TRUSTED_PREFIX) - 1) > +#define OVL_XATTR_ESCAPE_USER_PREFIX OVL_XATTR_USER_PREFIX OVL_XATTR_ESCAPE_PREFIX > +#define OVL_XATTR_ESCAPE_USER_PREFIX_LEN (sizeof(OVL_XATTR_ESCAPE_USER_PREFIX) - 1) > + > enum ovl_xattr { > OVL_XATTR_OPAQUE, > OVL_XATTR_REDIRECT, > diff --git a/fs/overlayfs/xattrs.c b/fs/overlayfs/xattrs.c > index b8ea96606ea8..27b31f812eb1 100644 > --- a/fs/overlayfs/xattrs.c > +++ b/fs/overlayfs/xattrs.c > @@ -4,6 +4,18 @@ > #include <linux/xattr.h> > #include "overlayfs.h" > > +bool ovl_is_escaped_xattr(struct super_block *sb, const char *name) > +{ > + struct ovl_fs *ofs = sb->s_fs_info; > + > + if (ofs->config.userxattr) > + return strncmp(name, OVL_XATTR_ESCAPE_USER_PREFIX, > + OVL_XATTR_ESCAPE_USER_PREFIX_LEN) == 0; > + else > + return strncmp(name, OVL_XATTR_ESCAPE_TRUSTED_PREFIX, > + OVL_XATTR_ESCAPE_TRUSTED_PREFIX_LEN - 1) == 0; > +} > + > bool ovl_is_private_xattr(struct super_block *sb, const char *name) > { > struct ovl_fs *ofs = OVL_FS(sb); > @@ -82,8 +94,8 @@ static int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char > > static bool ovl_can_list(struct super_block *sb, const char *s) > { > - /* Never list private (.overlay) */ > - if (ovl_is_private_xattr(sb, s)) > + /* Never list non-escaped private (.overlay) */ > + if (ovl_is_private_xattr(sb, s) && !ovl_is_escaped_xattr(sb, s)) > return false; > > /* List all non-trusted xattrs */ > @@ -97,10 +109,12 @@ static bool ovl_can_list(struct super_block *sb, const char *s) > ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size) > { > struct dentry *realdentry = ovl_dentry_real(dentry); > + struct ovl_fs *ofs = OVL_FS(dentry->d_sb); > ssize_t res; > size_t len; > char *s; > const struct cred *old_cred; > + size_t prefix_len; > > old_cred = ovl_override_creds(dentry->d_sb); > res = vfs_listxattr(realdentry, list, size); > @@ -108,6 +122,9 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size) > if (res <= 0 || size == 0) > return res; > > + prefix_len = ofs->config.userxattr ? > + OVL_XATTR_USER_PREFIX_LEN : OVL_XATTR_TRUSTED_PREFIX_LEN; > + > /* filter out private xattrs */ > for (s = list, len = res; len;) { > size_t slen = strnlen(s, len) + 1; > @@ -120,6 +137,12 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size) > if (!ovl_can_list(dentry->d_sb, s)) { > res -= slen; > memmove(s, s + slen, len); > + } else if (ovl_is_escaped_xattr(dentry->d_sb, s)) { > + memmove(s + prefix_len, > + s + prefix_len + OVL_XATTR_ESCAPE_PREFIX_LEN, > + slen - (prefix_len + OVL_XATTR_ESCAPE_PREFIX_LEN) + len); > + res -= OVL_XATTR_ESCAPE_PREFIX_LEN; > + s += slen - OVL_XATTR_ESCAPE_PREFIX_LEN; It's a bit hard to follow. how about: res -= OVL_XATTR_ESCAPE_PREFIX_LEN; name_len = slen - prefix_len - OVL_XATTR_ESCAPE_PREFIX_LEN; s += prefix_len; memmove(s, s + OVL_XATTR_ESCAPE_PREFIX_LEN, name_len + len); s += OVL_XATTR_ESCAPE_PREFIX_LEN + name_len; > } else { > s += slen; > } > @@ -128,11 +151,47 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size) > return res; > } > > +static char *ovl_xattr_escape_name(const char *prefix, const char *name) > +{ > + size_t prefix_len = strlen(prefix); > + size_t name_len = strlen(name); > + size_t escaped_len; > + char *escaped, *s; > + > + escaped_len = prefix_len + OVL_XATTR_ESCAPE_PREFIX_LEN + name_len; > + if (escaped_len > XATTR_NAME_MAX) > + return ERR_PTR(-EOPNOTSUPP); > + > + escaped = kmalloc(escaped_len + 1, GFP_KERNEL); > + if (escaped == NULL) > + return ERR_PTR(-ENOMEM); > + > + s = escaped; > + memcpy(s, prefix, prefix_len); > + s += prefix_len; > + memcpy(s, OVL_XATTR_ESCAPE_PREFIX, OVL_XATTR_ESCAPE_PREFIX_LEN); > + s += OVL_XATTR_ESCAPE_PREFIX_LEN; > + memcpy(s, name, name_len + 1); > + > + return escaped; > +} > + > static int ovl_own_xattr_get(const struct xattr_handler *handler, > struct dentry *dentry, struct inode *inode, > const char *name, void *buffer, size_t size) > { > - return -EOPNOTSUPP; > + char *escaped; > + int r; > + > + escaped = ovl_xattr_escape_name(handler->prefix, name); > + if (IS_ERR(escaped)) > + return PTR_ERR(escaped); > + > + r = ovl_xattr_get(dentry, inode, escaped, buffer, size); > + > + kfree(escaped); > + > + return r; > } > > static int ovl_own_xattr_set(const struct xattr_handler *handler, > @@ -141,7 +200,18 @@ static int ovl_own_xattr_set(const struct xattr_handler *handler, > const char *name, const void *value, > size_t size, int flags) > { > - return -EOPNOTSUPP; > + char *escaped; > + int r; > + > + escaped = ovl_xattr_escape_name(handler->prefix, name); > + if (IS_ERR(escaped)) > + return PTR_ERR(escaped); > + > + r = ovl_xattr_set(dentry, inode, escaped, value, size, flags); > + > + kfree(escaped); > + > + return r; > } > > static int ovl_other_xattr_get(const struct xattr_handler *handler, > -- > 2.41.0 >